Previous studies from this laboratory showed that combinations of butyric acid (BA) and RA are synergistic in promoting the differentiation of HL60 acute myeloid leukemia cells. BA, as a single agent, also is a powerful inducer of differentiation. While it has activity in many tumor cell systems in vitro it is not used in the clinic for treatment of malignancies primarily due to difficulties in achieving meaningful concentrations and a short plasma half-life. We have sought prodrugs which may circumvert these obstacles. Tributyrin is our lead candidate. Based on BA equivalents, it is as active as BA in differentiation assays in vitro. Furthermore, it gives potentially active concentrations of BA in vivo. Combinations of tributyrin and RA are synergistic in inducing differentiation of HL60 cells. These studies will therefore lead to a more complete understanding of the mechanism for the diverse effects of RA on mammalian cells and provide important preclinical information to optimize the use of BA as an anti-cancer agent.